Longevity gene responsible for robust blue organic materials employing thermally activated delayed fluorescence

The 3rd-Gen OLED materials employing thermally-activated delayed fluorescence (TADF) combine advantages of first two for high-efficiency and low-cost devices. Though urgently needed, blue TADF emitters have not met stability requirement for applications. It is essential to elucidate the degradation mechanism and identify the tailored descriptor for material stability and device lifetime. Here, via in-material chemistry, we demonstrate chemical degradation of TADF materials involves critical role of bond cleavage at triplet state rather than singlet, and disclose the difference between bond dissociation energy of fragile bonds and first triplet state energy (BDE-ET1) is linearly correlated with logarithm of reported device lifetime for various blue TADF emitters. This significant quantitative correlation strongly reveals the degradation mechanism of TADF materials have general characteristic in essence and BDE-ET1 could be the shared “longevity gene”. Our findings provide a critical molecular descriptor for high-throughput-virtual-screening and rational design to unlock the full potential of TADF materials and devices.


Fig. 9
The absorbance changes of the neat film of DMAC-DPS (a) and 5CzCN (b) before and after degradation.

Fig. 15
Chemical structures of 32 reported materials. The fragile bonds are labeled in red.

Fig. 16
The correlation between BDE-ET1 and device lifetime of 32 reported materials (35 points).

S14
Supplementary Discussions of materials exhibited exceptionally longer lifetime beyond the correlation.

Fig. 19
Chemical structures of host materials used in Supplementary Table 3. S19

Supplementary Discussions for numerical simulation. S20
Jablonski diagram of the exciton dynamics in TADF materials by photoexcitation. (Supplementary Fig. 20)

S20
Supplementary Discussions for the parameter choices in the numerical simulation.

S20
Supplementary description for the simulation. S21 The output simulation results of DMAC-DPS neat film. (Supplementary   Table 5)

S21
Supplementary discussions for the rationality of assumptions in numerical simulation.

S21
Supplementary discussions for the comparison between single exciton model and hot exciton models.

Solvent Choice.
We choose benzene as the solvent as it is (1) highly stable and difficult to occur photo-induced reaction in the experimental condition, (2) no absorption at the excitation wavelength of 340-380 nm, (3) moderate polarity, and (4) good solubility.

Degassed dilute solution and aerated concentrated solution
To further support the conclusion that blue TADF materials mainly degrade at T1 state, we conducted other 2 tests. We found that PL intensity of aerated solution is much weaker than that of degassed solution. For example, in DMAC-DPS, the PL intensity of aerated solution is only 16.3% of degassed solution ( Supplementary Fig. 3a). This is because the PL of TADF emitters include prompt part and delayed part. The delayed part comes from RISC of T1 excitons. Thus, once the T1 excitons are quenched by O2, the delayed PL will vanish, leading to the decreased PL intensity. On the other hand, the decreased PL intensity represents the decreased density of S1 excitons, so based on the unchanged PL intensity of aerated solution and solution with 2-BP in photo-degradation tests, we could not directly exclude the effect of difference in density of S1 excitons.
To solve this problem, we compared the degradation result of degassed dilute solution and aerated  Fig. 4b). As shown in Supplementary Fig. 4c, the initial PL intensity of aerated and concentrated solution with 1 mm light path is ~1/2 of that of diluted solution (228 and 415). Considering the fact that concentrated solution only has 1/10 emitting region of diluted solution. This PL intensity difference indicated that the density of S1 exciton in the aerated and concentrated solution would be larger than that of in degassed and diluted solution. Meanwhile, in the 1×10 mm 2 cuvette, the gradation of exciton concentration would be alleviated. Thus, the degradation results in Supplementary Fig. 4d that the PL intensity of concentrated solution almost did not change since the T1 excitons were quenched while the PL intensity of degassed dilute solution shows visible decay, strongly confirmed that TADF materials mainly degrade at T1 state once again.

Degradation tests of conventional fluorescence emitters
To confirm our conclusion from another aspect, we conducted the degradation tests of conventional shown in Supplementary Fig. 5, the PL intensity change of the two emitters is both below 5% neither in the degassed solution nor in the solution.
In summary, based on the contrast experiments on concentrated and dilute solution and the degradation tests of conventional fluorescence emitters, we further confirmed the conclusion that blue TADF emitters mainly degrade at T1 state rather than S1 state, laying the solid groundwork for our further explorations.    Fig 18a). It might originate from the high sensitivity of kRISC value to different measurement and calculation methods. This result demonstrated again that as for the "trait" of device lifetime, thermodynamic parameter BDE-ET1 can act as the intrinsic molecule "longevity gene", which is particularly valuable for high throughput virtual screening and material design. During "gene expression", "environment" such as carrier balance or host-guest interactions, would also have important influence on the "trait" of device lifetime because they could greatly affect the kinetic molecule parameters such as kRISC. For DBA-DI, TDBA-DI , and 5CzTrz, they all possess high kRISC values (6.21×10 6 s -1 , 1.08×10 6 s -1 , and 1.5×10 7 s -1 , respectively). Therefore, the longer operational lifetime of these 3 materials could also be partially attributed to the superior kRISC. Secondly With the assumption of knr,T = 0, kr,T=0 With the assumption of knr,S = 0, kr,T=0, S~p

Supplementary Note 2
In the above equations, P is the excitation power density of 1.48 and 2.10 mW cm -2 at 340 nm and 380 nm, respectively. λ is the   Material and data collection criteria: 1. EL emission peak<500 nm.
2. ET1 is obtained from the PL spectrum reported in the literature (the onset energy for spectrum with no fine structure and the first peak energy for spectrum with fine structure).

1.Supplementary descriptions of exciton dynamics and kinetic equations (1)-(3).
For S1 excitons formed through photo-excitation, they would directly decay via radiative (prompt and delayed fluorescence) or non-radiative (thermal dissipation) process, or convert into T1 excitons via intersystem crossing (ISC) process, or join singlet-singlet annihilation (SSA) / singlet-triplet annihilation (STA) process. T1 excitons formed through ISC from S1 would directly decay via radiative (phosphorescence) or nonradiative (degradation or thermal dissipation) process, or convert into S1 excitons via RISC process, or join STA/TTA process. Usually, SSA process consumes two S1 exciton and produce one S1 exciton, thus the coefficient of SSA process in equation (1) is 2-1=1 and STA process consumes one S1 exciton, one T1 exciton and produce one T1 exciton, thus the coefficient of STA process in equation (2) is 0 while in equation (1) is 1, and as for TTA process, it would consume two triplet exciton and produce γ S1 exciton and (1-γ) T1 exciton.
For all the materials studied in our work with ES1<2ET1, and ET2<2ET1, γ is 0.25. Of note, as the degradation tests were conducted under continuous UV illumination, the existence of polaron could be ignored. And just as the discussion in the introduction of main text, due to the final emission of TADF emitters is from singlet states not triplet states, in the simulation of TADF materials one could not mainly consider the decay process of triplet excitons like that in PH materials, but need full consideration of the complex decay process of singlet excitons as well, which brings more challenge to the simulations.

Supplementary discussions for the parameter choices in the numerical simulation.
In the parameters of simulation, since all the materials are blue emitters with high Photoluminescence Quantum Yield (PLQY), according to the energy gap law and PLQY= r,S /( r,S + nr,S + ISC ) , the nr,S could be ignored, and due to all the TADF emitters do not show room temperature phosphoresce (RTP), r,T is also neglectable. Values of r,S , ISC , RISC , and nr,T were obtained from experiments based on the method reported by Adachi et al. 5 . According to the literatures, SSA , STA , and TTA were set as 1×10 -12 cm 3 S21 s -1 , 1×10 -12 cm 3 s -1 , and 1×10 -14 cm 3 s -1 , respectively and remained constant in simulation, the rationality of this treatment would be verified later. QS and QT were set as 1×10 -9 cm 3 s -1 and 1×10 -11 cm 3 s -1 at first according to the related literatures and finely tuned during numerical simulation.

3.Supplementary description for the simulation.
First, we typed the equation (1)- (3) into Matlab, and then input aforementioned each rate constant. kQF was set as 1×10 -3 s -1 at first. Since in the photo-degradation test, the rate of exciton decay is much larger than that of quencher formation, an equilibrium would establish among these photophysical process. Thus, we could get the S ( ), T ( ), and Q ( ) at any time t according to the iteration through Matlab. The output simulation results are like data in Supplementary Table 4. According to the comparison between simulation result and experimental results, we optimized the value of kQF and made tiny adjustment to QS and QT to obtain wellfitted results (Fig. 4a). By this way, we could get the value of kQF, QS , and QT . 4.Supplementary discussions for the rationality of assumptions in numerical simulation.

4.1.
The assumption that the excitation intensity I is nearly constant during experiment. We compared the absorption spectrum of DMAC-DPS and 5CzBN before and after illumination, and found that their absorption intensity nearly unchanged ( Supplementary Fig. 9). This result demonstrates the decay of PL was originated from quenching by quenchers other than direct vanishing of emitters. So, it is reasonable that the excitation intensity I is nearly constant during experiment.

The rationality of the simulated kQS and .
We measured the prompt lifetime (τp) of fresh and aged film of each material and compared the experimental τp with those calculated based on the simulation results according to equation (4) and (5). The simulation results were well fitted with the experiment results ( Supplementary Fig. 10). As for the QT , values in our study are similar with those reported 20 by other researchers.

The verification of the deadly influence on material and devices degradation of very few quenchers
formed by irreversible bond cleavage. According to the simulation results, the rate constant of chemical S22 reaction quencher formation by irreversible bond cleavage, kQF~10 -3 s -1 is 8~10 magnitude smaller than those of photophysical processes such as kRISC ~10 5 s -1 and kf ~10 7 s -1 . It seems that the chemical process of bond cleavage should have little effect on the materials and device degradation. Of note, it is widely accepted that the degradation of OLEDs originates from exciton quenching by quenchers rather than the direct vanishment of emitters, which was evidenced by the little change of absorption spectrum of DMAC-DPS and 5CzBN before and after degradation. Based on such a small kQF, we got QT~1 0 -11 cm 3 s -1 , QS~1 0 -9 cm 3 s -1 , and Q~1 0 15 cm -3 . In the circumstances, the apparent rate constant of T1 excitons quenching, QT Q is ~10 4 s -1 , which is comparable with other rate constant of T1 excitons consumption, such as kRISC (~10 5 s -1 ). Likewise, for S1 excitons, the obtained QS Q~1 0 6 s -1 is comparable with kf (~10 7 s -1 ) etc. Thus, the very few quenchers formed by irreversible bond cleavage indeed could have deadly influence on degradation of material and devices.

5.Supplementary discussions for the comparison between single exciton model and hot exciton models.
Take TTA as an example, in the hot exciton model (TTA), equation (3)  Of note, we emphasized again that the neglect of SSA, STA, and TTA process is based on our experiment condition that the illumination intensity is ~2 mW cm -2 and the density of singlet and triplet excitons is 10 13 ~ 10 14 cm -3 , when the illumination intensity increases to ~ 10 3 mW cm -2 and the density of excitons increases to 10 16 ~ 10 17 cm -3 , the effect of SSA, STA, and TTA could no longer be ignored.